Parallel-connected dialing signal transmission-inhibiting device for data transfer over a telephone link

ABSTRACT

A parallel-connected dialing signal detection and transmission-inhibiting device is disclosed which detects dual-tone multi-frequency dialing signals across tip and ring conductors of a telephone link and switches in an a.c. load that attenuates the dialing signals by at least 30 dB. This prevents action in response to the DTMF signals by a central office servicing the telephone link, thereby allowing commands-data to be transmitted within the home telephone wiring. Such commands can be used to invoke a speed/auto dialing function, to provide an intercom, to control appliances, etc. To allow fast response, the controller charges a capacitor of the a.c. load by connecting it across the tip and ring conductors in response to an off-hook condition. Then it is disconnected to allow a dial tone to reach a user at the telephone. Further transparency is provided by the fact that a single interfacing capacitor connects both the dialing signal detector and a tone generator to the tip line. This reduces the loading on the tip line by the device during periods when users are conversing on the telephone link.

BACKGROUND OF THE INVENTION

Call function devices that work for all the telephones on the telephoneline, such as some speed dialers and call restrictors, are typicallyconnected serially between the user's telephone and the telephone linkto the central telephone office. In the operation of the speed dialer,dual-tone multi-frequency (DTMF) tones generated by the telephone arereceived by the device. If it is determined that these tones arerepresentative of a speed dialing code, the speed dialer accesses alookup table using the code as the address. A telephone number stored atthe address is then sent over the telephone link, and the telephone issubsequently connected to the link.

The keystrokes necessary to dial a full 11-digit telephone number, forexample, are cut to the number of keystrokes necessary to type the code,usually three or four keystrokes. Alternatively, if the DTMF tonesgenerated by the telephone are simply a telephone number that a userwishes to call, the speed dialer forwards the tones onto the telephonelink and connects the telephone to the link.

In this way, these devices add a new capability while still allowing theuser to dial normally.

The principle downside of serial-connected speed dialers is thedifficulty of installation. Serial connection can be made easily for onetelephone, but it presents difficulties for many telephones, since thereis often no modular jack available that allows a device to be pluggedbetween the central telephone office and all of the phones on a phoneline. This installation often requires an experienced technician, whichincreases costs.

Parallel-connected call restrictors have been proposed. These devicesare usually connected in parallel to telephones across the tip and ringlines of the telephone link to block certain dialing codes. The parallelconnection requires no special wiring because the parallel-connecteddevice can plug into a modular jack connected to the phone line and workfor all phones on the line. If the desired jack is being used by atelephone, a T-adapter can be used or the device can provide thisfunction. Parallel-connected speed dialers have also been proposed. Sucha dialer would be plugged into a modular jack to receive a speed dialingcode from other telephones on the link. The parallel-connected dialerthen generates a particular phone number that this code indicates.

SUMMARY OF THE INVENTION

Known parallel-connected call function devices, such as speed dialers orcall restrictors, have suffered from a number of problems. At some pointduring operation, the device should load the telephone link to ensurethat dialing signals generated by one of the telephones are not detectedby a central office serving the link. One method for achieving this isto place an a.c. (alternating current) load across the phone link's twoconductors, tip and ring. This a.c. load should severely attenuate thedialing signals on the line. Prior art specifications for the a.c. load,however, are inadequate for many real-world situations. Variability incentral office sensitivity, telephone network attenuation, and dialingsignal strength means that, in many common situations, the signalsgenerated by the telephone will be detected by the central office.

Moreover, in many cases the call function devices are not transparent tothe user. Some familiar aspects of telephone operation change withinstallation of the device, or the audio amplitude of the linkdeteriorates. For example, the user may no longer hear a dial tone orthe fidelity of the telephone link may be impacted.

In one aspect, the present invention is directed to a parallel-connecteddevice that inhibits the transmission of dialing signals over atelephone link. An a.c. load, preferably a series-connected capacitorand resistor, is provided to inhibit transmission by attenuation of thesignals. Specifically, the signals are attenuated by at least 30 dB.This attenuation should ensure that the dialing signals will notinitiate action by central offices meeting accepted specifications. Inspecific embodiments, higher attenuations are preferable. If oneconsiders real-world central offices and worst-case DTMF sensitivities,attenuation of at least 38 dB is preferred.

In other aspects, the design and operation of the invention provide fortransparency to the user. For example, in the time between the phoneinitially going off-hook and the transmission of the first DTMF signal,the user at the telephone is able to hear a normal dial tone with thepresent invention. This operation is deceptively difficult to achievesince the a.c. load attenuates the dial tone. This fact suggests thatthe load should be connected across the telephone link in response tothe detection of the start of the first dialed digit. On the other hand,an a.c. load that will properly attenuate the dialing signals and thatis charged by d.c. current from the central telephone office, only whenthus connected, may take too long to charge up. A certain minimumvoltage required by the phone's circuitry for DTMF signal generation maynot be available for at least a portion of the time while the capacitorcharges so the telephone may not be able to continue to generate theDTMF signal for proper detection. In the present invention, this problemis solved by connecting the a.c. load across the telephone link when atelephone's off-hook status is first detected. This charges a capacitor,the primary a.c. load. Subsequently the a.c. load is disconnected,allowing the dial tone to reach the user. The a.c. load is thenreconnected only when the user begins to dial a telephone number orother code.

Further transparency is provided in certain embodiments by using asingle interfacing capacitor to connect both a DTMF detector and a tonegenerator to the tip line. This reduces the loading on the tip line bythe device during periods when users are conversing on the telephonelink, thus preserving the audio quality of the telephone link, and alsoreduces the manufacturing cost of the device.

The invention may operate as a speed dialer generating a telephonenumber in response to a code entered at the telephone number pad. Inother applications, the invention may select a telephone carrier inresponse to the dialed telephone number and forward the telephone numberto the selected telephone carrier. This feature is helpful forintra-LATA toll calls in which long distance carriers often have betterrates. Still other applications for the invention include transmittingDTMF signals encoding a credit card number for billing purposes when apredetermined dialing code is entered at the telephone. In anothersituation, the invention functions as a toll call restrictor by onlyallowing certain calls, such as local and "800" telephone numbers, to besent to the central office.

In still other embodiments, the invention relies on an equalizingnetwork to provide a dialing signal to the dialing signal detector.Because of the network, these dialing signals have the same input levelwhether or not the switchable load is connected to or disconnected fromthe telephone link. The equalizing network voltage divides the signalsin response to the state of the load to help ensure that signals of thesame magnitude are presented to the detector regardless of the state ofthe load.

According to another aspect, the invention concerns a system fortransmitting information over telephone wiring of a home or office, forexample. Such a system has a detector for identifying dialing signalsacross the tip and ring conductors of the telephone wiring. Thesedialing signals are typically generated by a conventional telephone alsoconnected to the wiring. A switchable load selectively connects to thetip line to attenuate the dialing signals to prevent action by a centraloffice connected to the telephone wiring, but a controller detects theseblocked signals. If they are simply a telephone number, they areforwarded to the central office as described previously. In contrast,when it is determined through the formatting that the dialing signalsare intended for the system, they are interpreted as commands.

In specific embodiments, the commands are used to control an appliance,such as a thermostat, lights, etc. In other cases, the controller usesthe commands to select an intercom message, or to broadcast a spokenmessage from the telephone as an intercom message.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionis shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a block diagram of a parallel-connected data transfer systemfor a telephone link including a device for inhibiting transmission ofdialing signals to a central office while allowing detection of thesignals according to the present invention;

FIG. 2 is a flow diagram illustrating a method of operation of theinventive system;

FIG. 3 illustrates an additional application for the inventive system,i.e., a device for transmitting information within a given telephonelink to control home electronic devices or provide an intercom function;

FIG. 4 is a circuit diagram of the data transfer system of FIG. 1; and

FIGS. 5A and 5B show the principle paths between tip and ring when therelay SW1 is open, FIG. 5A, and closed, FIG. 5B, to show the operationof the equalizing network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a parallel-connected data transfer system100 for a telephone link 10, which has been constructed according to theprinciples of the present invention. The system 100 is controlled by amicroprocessor 400 that operates by a clock signal from clock 410 andaccesses data from or stores data to an optional non-volatile RAM 420.

An off-hook detector 500 is connected to monitor tip and ring lines20,22 of the telephone link 10. It provides an off-hook signal on signalpath 212 to the microprocessor 400 in response to one of the telephones30 or 32 going off-hook. The off-hook signal indicates to themicroprocessor 400 that dialing and other a.c. signals may appear. Theoff-hook detector 500 additionally functions as a ring detector. Signalpath 212 will vibrate in response to incoming ring signals to indicateto the microprocessor 400 that there is an incoming call.

A tone/signal generator 300 is constructed to generate at leastDTMF-type dialing signals under the control of the microprocessor 400.In some implementations, however, it may be desirable to additionallygenerate modem signals or other a.c. signals. The tone generator 300 iscoupled to the tip line 20 through an interfacing capacitor 250.

A switchable a.c. load 200 is selectively connectable across the tip 20and ring lines 22 of the telephone link 10 to severely attenuate signalssent between the telephones 30, 32 and the central office 5. Thisoperation is provided by a relay SW1 that is under the control of themicroprocessor 400. When the relay SW1 is closed by the microprocessor,a series connected capacitor C1 and resistor R1 provide a low impedancepath between tip and ring. Resistor R1 and capacitor C1 are selectedsuch that DTMF signals, for example, generated by telephones 30 or 32are attenuated to a level at which the central office 5 will notrecognize the tones as valid.

An a.c. signal amplifier 218 transmits dialing signals on the phone link10 to a DTMF/tone detector 210. Two potential signal paths from the tipline 20 to the detector 210 via the amplifier are provided. TheDTMF/tone detector 210 is continuously connected to tip through theinterfacing capacitor 250, an equalizing network 216, and a.c. signalamplifier 218. Using the shared capacitor 250 for the tone/signalgenerator 300 and the DTMF/tone detector 210 decreases residual loadingcaused by the connection of the system 100 to the telephone link 10.This ensures that the audio volume of the link is not seriously reducedby the system 100. When the relay SW1 is in a closed position, thevoltage across a resistor R1 is applied through the equalizing network216 to the a.c. signal amplifier 218. The equalizing network 216functions to ensure that, regardless of the relay's state, the amplitudeof DTMF signals from telephones 30,32 presented to the a.c. signalamplifier 218 is substantially the same and detected by detector 210.

The attenuation provided by the switchable a.c. load 200 must be atleast 30 dB to ensure that the central office 5 will not respond undermost line conditions, although an attenuation of at least 38 dB isrequired to work under a broader spectrum of situations. To achieve theattenuation, the switchable a.c. load 200 comprises the microprocessorcontrolled relay SW1 and a 0.22 ohm resistor R1 in series with a 220 μFnon-polarized capacitor C1. The capacitor C1 ensures that d.c. (directcurrent) voltage supplied by the central office 5 across tip and ring isunaffected once the capacitor C1 has reached a steady-state d.c.condition.

The selection of the resistor R1 and capacitor C1 is driven by thefollowing analysis. The signaling chapter of "BOC Notes on the LECNetwork--1990" states that a central office DTMF receiver shouldregister DTMF digits with a power per frequency of -25 to 0 dBm. Thereceiver should not respond if either frequency component of the signalis below -55 dBm into 900 ohms. Thus, the receiver is required to dealwith a minimum range of 25 dB per tone and is not allowed to permit arange of 55 dB per tone.

The specifications are reflected in commercially available DTMF receiverchips. The Mitel MT8870D integrated DTMF receivers are claimed to be"central office quality"; and they accept a DTMF low group or high grouptone of -29 to +1 dBm. The Mitel MT3170B family of wide dynamic rangeDTMF receivers are suitable for end-to-end signaling; and a valid inputsignal level is from -50 to 0 dBm for either of the two DTMF tones. Itis unclear whether any central office switches ever use such a sensitiveDTMF receiver, but there is no guarantee that they are never used.

Thus, a typical central office DTMF receiver will detect amplitudes overat least a 30 dB range per frequency, and some DTMF receivers may detectamplitudes over a range of as high as 50 to 54 dB. AT&T central officeswitches typically reject DTMF tones less than -38.2 dBm per tone. Aparallel dialer, or other parallel device of the present invention, isdesigned in view of the above specifications to prevent DTMF tones frombeing detected at the central office.

Further, it is typically not known whether a given telephone will be onthe high end or low end of its permitted signal strength and it istypically not known exactly how the telephone link and central officecircuitry will affect the DTMF signal. The parallel device might, forinstance, be in a situation with a strong signal source, low-attenuationlocal loop, and high-sensitivity DTMF receiver. The parallel deviceshould be designed to work over the highest practical range ofsituations. As a result, an attenuation of at least 30 dB is necessaryto enable operation under most circumstances. At least a 38 dBattenuation will ensure operation with AT&T central office switches.

Simply selecting a very high attenuation, however, is not a satisfactorysolution. If relay SW1 is closed, increasing attenuation has theundesirable effect of increasing the time required to detect that aphone has been hung up. This effect limits the maximum attenuation.

In the preferred approach implemented in FIG. 1, the goal is toattenuate all frequencies in the high frequency group of the DTMFsignals by at least 45 dB. The worst case occurs for the DTMF frequencyof 1209 Hz, the lowest high-band DTMF frequency. At this frequency, onecommercially available non-polarized 220 μF capacitor C1 has anequivalent series resistance of at most 0.8 ohms and afrequency-dependent impedance of 0.6 ohms. Accounting for the resistancefrom relay SW1, from the one-foot phone cable connecting the dialer tothe phone jack, and from any stray circuit and component resistances,resistor R1 should have a resistance of approximately 0.22 ohms.Clearly, the size of resistor R1 and capacitor C1 can be varied but thetotal amplitude of the complex impedance of the dialer and its cableshould be kept below 1.36 ohms to achieve 45 dB attenuation in theclosed-relay condition.

It is important to point out that the length of the cable and telephonewiring connecting the dialer to the path between the off-hook phone andthe central telephone office does affect the attenuation, since a longerpath generates higher resistance and lower attenuation. Consequently,the best place to put the dialer is normally at the modular jack nearestthe place where the phone wiring enters the premises. This is often notrequired, depending on other factors like the length of the wiring inthe home, the strength of the DTMF generators in the phones, theattenuation of the path to the central telephone office, and thesensitivity of the central office DTMF receiver. In any event, even whenconnecting at this point is required, it is normally much easier than aseries connection.

In one embodiment, the system 100 is configured to operate as a storeand forward dialer. The goal of this operation is to provide therequired functionality yet maximize the transparency to the user. Oneaspect of transparency is ensuring that the dial-tone, the tone that onetypically hears when the telephone receiver is first picked-up, is notattenuated by the system 100. The a.c. load 200 will, however, attenuatethis tone when the relay SW1 is closed. Therefore, the relay SW1 shouldonly be closed when the system 100 begins to detect the first DTMFsignal from the off-hook telephone. Closing the relay at this point willstop the dial tone as is typical. A problem arises in that to properlyload the phone link 10 to ensure that the central office 5 does notrespond to the telephone-generated dialing signals, resistor R1 is smalland capacitor C1 is large, resulting in a low impedance. If thepremises' phones had been on-hook long enough to discharge voltage oncapacitor C1 to near zero, some time will be required to recharge thecapacitor C1 once the a.c. load 200 has been connected across the phonelink. At the start of this time only a small d.c. voltage will appearacross the tip and ring conductors of the off-hook phone. While the d.c.voltage is low, the phone may be unable to generate DTMF signals, whichmay leave an insufficient time to detect the two tones of the DTMFsignals.

The time required to adequately charge the capacitor C1 can beestimated. In the preferred embodiment, capacitor C1 is approximately220 μF. When the relay SW1 is closed, C1 is charged from the centraltelephone office battery (typically 50 volts DC) through the phoneline's resistance of typically 400 to 2000 ohms, resulting in a timeconstant of 88 to 440 milliseconds. The time required for capacitor C1to charge to a high enough voltage to permit DTMF generation istypically about one-sixth of this time constant, or about 74milliseconds maximum.

FIG. 2 illustrates a method of operation of the invention when operatingas a store and forward dialer, which solves the identified problems.According to the invention, in an initial state, the relay SW1 is in anopen condition but the off-hook detector 500 is active to determine whenone of the telephones 30 or 32 goes to an off-hook condition, step 610.This represents a standby mode in which the system 100 is essentiallydormant.

When one of the phones is off-hook, the off-hook detector 500 sends anoff-hook signal to the microprocessor 400 via signal path 212, in step620. The microprocessor 400 immediately responds by closing relay SW1,step 625. The microprocessor holds the relay SW1 closed for at least 74millisecond, but preferably 200 milliseconds, in step 630. Then afterthe expiration of this time period, the relay SW1 is opened, step 635.Transparency is provided by the fact that the dial tone is only blankedfor the first 200 milliseconds after a user picks up the telephone'sreceiver. It is assumed that the typical user would normally not carewhether the dial tone was blanked for this period. It is further assumedthat during this short capacitor charging period, the typical operatorwill not yet have begun dialing.

The connection and disconnection in steps 625, 630, and 635 charges thecapacitor C1 to ensure that a sufficient voltage drop will be presentacross the off-hook phone during DTMF signaling to enable the generationand identification of those signals.

Alternatively, the relay SW1 does not need to be opened in step 635. TheDTMF detector 210 can of course detect any DTMF signals across resistorR1 when the relay is closed. With the relay SW1 closed, however, thedial tone will also be attenuated by the a.c. load 200.

In step 640, the DTMF detector 210 waits for DTMF dialing signals toappear across the telephone link 10. Any signals will reach the DTMFdetector 210 through the interfacing capacitor 250 and the equalizingnetwork 216 in step 640; no signals reach the detector through thecapacitor C1 since relay SW1 is open.

When the microprocessor 400 is provided with an indication of a DTMFsignal on conductor 222 from the DTMF detector 210 in step 645, themicroprocessor 400 quickly closes the relay SW1 in step 650. This placesthe a.c. load 200 on the tip line 20 which attenuates the DTMF signalsto a level at which the central office 5 will not respond. This relayclosing must occur quickly to ensure that the central office 5 does notinterpret the signal as a valid DTMF signal. "BOC Notes on the LECNetwork--1990" states that a DTMF receiver must reject a DTMF tone thatis less than 23 milliseconds; so the capacitor C1 must be switchedacross the phone link in less than this time. Once capacitor C1 isswitched in, it will dramatically reduce the amplitude of the a.c.signal sent to the central office's DTMF receiver. Since capacitor C1has been precharged, its DC voltage will not lower the off-hook phone'svoltage enough to prevent DTMF dialing. Even short DTMF signals can bedetected across a load resistor R1 of the a.c. load 200 by the DTMFsignal detector 210. Therefore, the preferred embodiment, withoutchanging the apparent operation of the phone, ensures that the firstDTMF signal does not reach the central office 5.

As subsequent dialing signals are generated by the off-hook telephone,they are consecutively detected and stored in the microprocessor 400,step 655. Then, once the microprocessor 400 has detected a suitablesequence of DTMF dialing signals, the relay SWI is opened, step 660.Now, system 100 can take one of three courses of action. First, if thenumber which was dialed is simply a valid telephone number that shouldbe dialed as is, step 665, the microprocessor 400 regenerates the DTMFdialing signals on the tip line 20 by generating the appropriate tonesthrough the tone generator 300, step 670.

Alternatively, if the detected DTMF tones represent a speed dialingcode, step 675, the microprocessor 400 treats the tones as an addressfor a look-up table stored in the microprocessor's non-volatile RAM(NVRAM) 420, step 680. This address location stores the telephone numberwhich the user wishes to dial. The microprocessor 400 consecutivelysends the DTMF dialing signals encoding this telephone number throughthe tone generator 300, step 685.

The DTMF signals generated by the telephone can also be commands forprogramming the dialer, step 690. For example, if two pound signals (##)are first detected, followed by the digit S (7) for "Store", followed bya pound signal (#), followed by a first sequence "name" of two numbers,followed by a second pound signal (#), followed by a second sequence"number" of numbers, followed by a third pound signal, this can be anindication to store at the shorthand "name", the "number", in step 695.As a result, when the user enters in a dialing command like poundfollowed by the name followed by a second pound, the system 100 willspeed dial the corresponding telephone number. Other programmingcommands can change the length of DTMF tones or the intervals betweenDTMF tones, or otherwise program information into the dialer.

Another application of the present inventive system is as a longdistance carrier selector for intra-LATA (Local Access and TransportAreas) toll calls. Commonly, intra-LATA calls default to the localtelephone company. However, many times the long distance carrier canprovide a less expensive calling rate. These long distance carriers canbe typically accessed by leading the telephone number with the accesscode of the desired long distance carrier--for instance, 10ATT. Thisrequires a person to spend time and energy remembering and dialing extradigits.

According to the present invention, the microprocessor 400 receives thetelephone number that the user dialed and analyzes it, as in steps610-665. From this information, the system 100 makes the decisionwhether or not the call should default to the local telephone company orwhether to route the call through an alternate carrier. In the casewhere the local telephone company is desired, the system simplyregenerates the desired telephone number. When the long distance carrieris desired, the system first generates that carrier's access code, suchas 10ATT, followed by the DTMF signals corresponding to the desiredtelephone number, such as 1 followed by a 10-digit number.

Still another application of the system 100 is as a billing codegenerator. It is common for users to make credit card calls. Thisrequires that the user first dial an access code or the "800" number ofthe long distance carrier. When connected to the carrier, the user dialsthe credit card number for billing purposes. Typically this is afourteen digit code. The system 100 can be configured to automaticallydial this number when some predetermined code is entered at thetelephone key pad. As before, this code is not transmitted to thecentral office. The system receives the code while loading the tip line.Afterwards, the system generates the proper credit card number inresponse to the code.

The system 100 may also be used to restrict toll calls. In manysituations, the owner of a telephone may want to provide telephoneaccess but restrict the user from making certain toll calls and thusincur expenses on behalf of the owner. Telephone numbers beginning withcertain area codes and/or international dialing prefixes should beblocked from reaching the central office. Local phone numbers with orwithout area codes, along with "800" toll-free numbers, should be passedthrough to the central office. The system 100 provides thisfunctionality.

When operating as a toll call restrictor, the microprocessor 400receives the telephone number that the user dials as in 610-665 of FIG.2. A determination is then made as to whether the user should be allowedto place the call. Typically, if the phone number will result in chargesto the telephone's owner, the number will not be forwarded to thecentral office 5. In other situations, it is possible that onlyinternational or "900" numbering will be blocked, probably where anemployee-employer relationship exists. In the alternative case, when thephone number is one that the user may dial, the number is transmitted tothe central office by the system 100 as described in step 670 of FIG. 2.

Referring to FIG. 3, an additional application for the above-describedinventive system is a device for transmitting data entirely within agiven telephone link 10. In such a configuration, other home appliances720, lights 730, thermostats 710, etc. are connected to the telephonewiring via telephone jacks in a home 705, for example. These connectionsare made via parallel-connected data transfer systems 100 as shown inFIG. 1. Then, by dialing with an off-hook telephone 30, a user canessentially call these other devices using only the home's wiring toissue commands to them. And as described previously, these dialingsignals are attenuated to avoid action by the central office 5.

For example, to program thermostat 710, the user takes telephone 30off-hook and dials a code that would be recognized by system 100 of thethermostat 710. The system 100 for thermostat 710 includes switchablea.c. load 200 and DTMF/tone detector 210 of FIG. 1. As a result, thedialing signals are blocked and not detected by the central office 5.They are, however, detected and interpreted by the thermostat 710 assignals for programming the thermostat. In this way, telephone wiringwithin someone's premises is used to send information from a telephone30 or other DTMF-generating device to a DTMF-receiving device 710-740 onthe premises 705. That receiving device 710-740 processes the DTMFinformation and possibly performs some function as adjusting thethermostat, turning lights on and off, or controlling appliances. In thealternative case where the user has dialed a telephone number, theappropriate signals are forwarded to the central office 5 as describeearlier.

A similar approach can be used to provide an intra-home intercom. Thisembodiment is designed to handle two modes of operation in each of twosituations. The situations are: 1) where a telephone call is takingplace on the telephone link 10 to another party via the central office5; and 2) where a telephone call is not taking place.

In the first situation, a single or series of DTMF tone can be used toactivate the intercom device 740 connected to the telephone wiring.These tones may be attenuated by the intercom 740. This function is notcritical here, however, since typically the central office 5 does notreact to any tones once a call has been placed. The intercom 740 has oneof two possible modes of operation. In response to the activating tones,prerecorded messages can be audibly reproduced at the intercom device740. Such a message could be "Alecia, please come to the phone." In thismode, a number of messages are available; the tones select the one to bereproduced. Alternatively in the other mode, the a.c. signal associatedwith the conversation on the telephone link 10 can be audibly broadcastby the intercom 740. In this implementation, a predetermined sequence oftones is used to start the broadcast by the intercom 740. Then when theuser wishes to terminate the operation of the intercom 740, anotherpredetermined sequence of tones is entered.

The situation in which a call is not occurring is similar but withmodification. Where, the prerecorded message mode is used, the intercomdevice 740 must attenuate the DTMF tones used to activate it. Otherwise,a call could unintentionally be placed. If the intercom device 740 is toaudibly broadcast a conversation on the telephone link 10, the operationis slightly more complicated. Here again, DTMF tones used to signal theintercom 740 to begin broadcasting the conversation should be attenuatedto avoid action by the central office. However, if all DTMF tones wereattenuated in the usual fashion, an unpleasant dial tone would interferewith the communication. Thus, a single or few tones should betransmitted to the central office so that the dial tone is turned off.

FIG. 4 is a detailed circuit diagram of the inventive system 100 showingone possible configuration. Generally, the system 100 connects to thetip 20 and ring 22 lines via a telephone jack 270. Ring 22 is shownproviding the reference potential throughout the circuit. Fuse F1provides some protection for the circuit in the case of improperconnection or a lightning strike on the phone line 10. A varistor 259shorts out large voltage surges between the lines 20,22.

The off-hook detector 500 operates by sensing the voltage between thetelephone link conductors tip 20 and ring 22. During on-hook conditions,the voltage across the telephone link 10 is approximately 50 Volts whenthe phone jack 270 is wired correctly, and approximately -50 volts whenthe wiring is reversed. Because some phone jacks are wired incorrectly,C1 is a non-polarized capacitor in the preferred embodiment. When atelephone goes off-hook, the voltage across tip 20 and ring 22 drops toa much lower voltage, typically about 10 volts. Comparator A1 compares areference voltage VR2 received at a non-inverting input terminal (+) toa voltage at an inverting terminal (-) which is indicative of thevoltage between tip and ring. Protective zener diode Z1 connects to thering line 22, the circuit's reference potential. Resistances R7 and R26are chosen such that the output of comparator Al is a steady low whenthe voltage across tip and ring is indicative of a normal on-hookcondition (i.e., tip positive with respect to ring) and a steady highwhen at least one of the phones is off-hook.

In cases of improper installation, the tip line 20 and ring line 20 maybe reversed at jack 270. A second comparator A2 is provided to handlethis situation. Specifically, the second comparator A2 compares thedivided voltage of resistor R26 to the ring conductor 22. If the dividedvoltage remains negative, there is an on-hook condition with tip andring lines crossed. The amplifier A2 indicates the condition bygenerating a negative output.

Diode D1 connects the output of comparator A1 to the microprocessor 400and D2 connects the comparator A2 to the microprocessor 400. ResistorR29 provides a connection to +5 Volts, the result being a steady logiclow input on line 212 to the microprocessor 400 if all phones areon-hook, and a steady logic high input if any phone is off-hook.

The off-hook detector 500 also facilitates ring detection. An incomingring signal causes the line 212 to the microprocessor 400 to fluctuatein a characteristic way that the microprocessor 400 detects as anincoming ring.

The interfacing network 216 is designed to ensure that the voltageamplitude of a DTMF signal received by the a.c. amplifier 218 throughcapacitor C6 and resistor R10 is substantially the same whether or notthe relay SW1 is in an open or closed condition. An additionalconsideration is the fact that resistor R36 should be large to minimizeattenuation due to the dialer when relay SW1 is open, as for example,when parties are talking on the telephone link 10 and the system 100 isessentially inactive. The interfacing network 216 comprises resistors R9and R36. Resistor R36 provides a connection between the tip line 20 andthe a.c. amplifier 218 through the interfacing capacitor 250 when therelay SW1 is open. When the relay SW1 is closed, resistor R9 alsoconnects the a.c. amplifier 218 to the tip line 20 via capacitor C1 ofthe a.c. load 200.

FIG. 5A is an equivalent circuit used to calculate the input signalentering the a.c. amplifier 218, at capacitor C6, when the relay SW1 isopen. R5 connects through R27 to the output of an operational amplifierA4, which is an a.c. ground when the operational amplifier A4 isquiescent as it is when DTMF tones are being detected. The effect of R27is negligible because its resistance is much lower than the resistanceof resistor R5. The effect of C6 and R10 is negligible since R10 is muchgreater than R9 plus R1. The effect of capacitor 250 is negligiblebecause its impedance is much less than that of resistor R5. The effectof resistor R1 is negligible because it is much smaller than R9. FIG. 5Bis an equivalent circuit when relay SW1 is closed. Here the effect ofresistor R36, resistor R5, and capacitor 250 is negligible since R36 ismuch greater than the series impedance of C1 and R9. The effect of R9 isnegligible because it is much smaller than resistor R10.

To ensure that the amplitude of the DTMF signals received at the a.c.amplifier 218 are substantially the same regardless of the state ofrelay SW1, a simplifying assumption is made that the DTMF generator onthe telephone link 10 may be modeled as an a.c. source with a seriesimpedance of 600 ohms, and that this drives the phone line 10 modeled asa 600 ohm load as well as the dialer in parallel with the 600 ohm phoneline. Assuming that R36 is much larger than R9, for the relay-open caseof FIG. 5A, the input to a.c. amplifier 218 will be the DTMF tone'samplitude times this number K: K=(R9/R36)*RP/(RP+600), where RP is theresistance of three resistors in parallel--600 ohms, R5, and R36. Kequals 0.000371 if R9 is 10 ohms, R36 is 11 Kohms, and R5 is 1527 ohms.

For FIG. 5B, if R1 is much less than 600 ohms, the input to the a.c.amplifier 218 will be the DTMF tone's amplitude times R1 divided by 600.If R1 is 0.22 ohms, this is 0.000367, which is close to the ratio0.000371 for FIG. 5A. If these conditions are met, then the voltagesupplied to the a.c. amplifier 218 should be substantially the samewhether or not relay SW1 is open or closed.

The a.c. amplifier 218 comprises a comparator A3 that receives thesignals from the interfacing network 216 at an inverting input terminal(-) through the series connected capacitor C6 and resistor R10. Anon-inverting input terminal (+) receives a reference voltage VR1.Negative feedback is provided through a parallel connected capacitor C7and resistor R11. The output terminal of comparator A3 of the a.c.amplifier 218 is connected the DTMF/tone detector 210.

The DTMF/tone detector 210 can be easily realized in an integratedcircuit. The preferred embodiment, however, has two bandpass filters252,258, one for the low DTMF group and one for the high group. Theoutput of each filter is fed into a separate zero-crossing detector254,256 with hysteresis. The output of each of these zero-crossingdetectors is fed into the microprocessor 400, which counts zerocrossings and analyzes the results. This approach reduces costs andallows the same filters to be used for other a.c. signal analysis, suchas detection of dial tones and other call progress tones such as a longdistance company's computer-generated tones from its switches.

In more detail, a high-tone bandpass filter 258 and the low-tonebandpass filter 252 receive the output of the a.c. amplifier 218. Thehigh-tone bandpass filter 258 isolates the high tones of the DTMFsignals. Possibly, this filter is an active bandpass filter. Thelow-tone bandpass filter 252 may also be an active type filter andisolates the low tones of the DTMF signals. The outputs from thehigh-tone bandpass filter 258 and the low-tone bandpass filter 252 arefed into separate zero-crossing detectors 254 and 256, respectively.Both zero-crossing detectors 254 and 256 have hysteresis for noiseimmunity. The outputs of the detectors 254 and 256 are sampled by themicroprocessor 400. By analyzing the intervals between zero crossings ofboth the high-tones and the low-tones of the DTMF signals on the tipline 20, the microprocessor 400 discriminates the different frequenciesin signals.

The tone/signal generation circuit 300 is basically a summing amplifierwith high-frequency rolloff. It comprises a resistor network RN,blocking capacitor C8 and an amplifier A4 with feedback resistor R25 andcapacitor C14. The microprocessor 400 generates 6-bit digitalrepresentations of a wave form having the desired frequency make up suchas, for example, a dual-tone multi-frequency dialing signal. This 6-bitdigital value is converted to the actual a.c. signal by tone/signalgeneration circuit 300 that acts as a summing amplifier of the weighted6-bit samples, with the lowest order bit having the lowest weight, thesecond lowest order bit having twice the weight of the lowest order bit,etc., with each higher order bit having twice the weight of the bitbelow it in significance. The resulting a.c. current is amplified byamplifier A4 by an amount proportional to R25, and rolled off for highfrequencies by capacitor C14. The output of comparator A4 is connectedto the tip line 20 via resistors R27 and R5 through the interfacingcapacitor 250. Zener diode Z2 and resistor R27 are provided to helpprotect amplifier A4 from large, fast transitions of the tip-ringvoltage.

The effect of the system 100 on the audio quality of telephone link 20should be minimized. This goal is facilitated by making the impedancefrom the series elements R5, R27, and capacitor 250 as large aspossible, but this large impedance makes it more difficult for theamplifier A4 to generate sufficiently large dialing signals. As aresult, the supply voltage of the comparator A4 is a full 15 Volts, andthis feature allows large voltage outputs to counter the effects of theintentionally large series impedance.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

We claim:
 1. A parallel-connected dialing signal detection and transmission-inhibiting device, comprising:a detector for identifying dual-tone multifrequency dialing signals across the tip and ring conductors of a telephone link; a switchable load selectively connectable across the tip and ring conductors for attenuating the dialing signals by at least 30 dB; a tone generator for generating dialing signals on the telephone link; a shared capacitor for coupling the tone generator and the detector to the tip line; and a controller for controlling a call and the switchable load in response to the identified dialing signals, the controller detecting dialing signals, generating a sequence of digits to dial in response to the detected dialing signals, and controlling the tone generator to transmit dialing signals corresponding to the sequence of digits to a central office controlling the telephone link.
 2. A device as claimed in claim 1, wherein the switchable load attenuates the dialing signals by at least 38 dB.
 3. A device as claimed in claim 1, wherein the switchable load comprises a capacitor load in series with a resistance.
 4. A device as claimed in claim 3, wherein the capacitor load comprises a non-polarized capacitor.
 5. A parallel-connected dialing signal detection and transmission-inhibiting, device comprising:a detector for identifying dual-tone multifrequency dialing signals across the tip and ring conductors of a telephone link; a switchable load that is connectable across the tip and ring conductors for attenuating the dialing signals by at least 30 dB; a controller for controlling a call and the switchable load in response to the identified dialing signals; and an equalizing network for enabling the detector to detect dialing signals across the switchable load when the switchable load is connected across the tip and ring conductors and when the switchable load is not connected across the tip and ring conductors.
 6. A device as described in claim 5, wherein the equalizing network presents dialing signals across the tip and ring conductors to the detector at substantially the same magnitude whether or not the switchable a.c. load is connected across the tip and ring conductors.
 7. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link having tip and ring conductors, the device comprising:a dialing signal detector for identifying dialing signals across the tip and ring conductors of the telephone link; a switchable a.c. load connectable across the tip and ring conductors for attenuating dialing signals transmitted across the tip and ring conductors; a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link; and an off-hook detector for generating an off-hook signal in response to a telephone on the telephone link going off-hook, wherein the controller pre-charges a capacitor of the a.c. load by connecting it across the tip and ring conductors in response to the off-hook signal and then disconnects the capacitor to allow a dial tone of substantially normal amplitude to reach a user at the telephone.
 8. A device as described in claim 7, wherein the controller detects the dialing signals, selects a telephone carrier in response to a telephone number encoded in the dialing signals, and forwards the telephone number to the selected telephone carrier.
 9. A device as described in claim 8, wherein the controller forwards the telephone number to the selected telephone carrier by preceding the telephone number with an access code of the selected telephone carrier.
 10. A device as described in claim 7, wherein the controller detects dialing signals, determines if a telephone number encoded by the dialing signals is an allowed telephone number, and forwards the telephone number to the central office only if it is allowed.
 11. A device as described in claim 10, wherein allowed telephone numbers comprise non-toll telephone numbers.
 12. A device as described in claim 7, wherein the controller connects the a.c. load to the telephone link to prevent all but the first part of the first dialing signal from reaching the central office servicing the telephone link.
 13. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link having tip and ring conductors, the device comprising:a dialing signal detector for identifying dialing signals across the tip and ring conductors of the telephone link; a switchable a.c. load connectable across the tip and ring conductors for attenuating dialing signals transmitted across the tip and ring conductors; a tone generator for generating dialing signals on the telephone link; and a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link, the controller detecting the dialing signals, accessing a speed-dialing number in response to the detected dialing signals, and controlling the tone generator to transmit dialing signals corresponding to the accessed number to a central office controlling the telephone link.
 14. A method for inhibiting transmission of dialing signals on a telephone link to a central office, the method comprising:charging a capacitor of an a.c. load; after charging, detecting for dialing signals on the telephone link from a telephone; and attenuating the dialing signals by connecting the charged a.c. load to the telephone link to prevent action by the central office in response to the dialing signals.
 15. A method as described in claim 14, further comprising:accessing a list of speed-dialing numbers in response to the detected dialing signals; determining a telephone number indicated by the dialing signals; and transmitting dialing signals corresponding to the telephone number to a central office controlling the telephone link.
 16. A method as described in claim 14, further comprising:selecting a telephone carrier in response to a telephone number encoded in the dialing signals; and forwarding the telephone number to the selected telephone carrier.
 17. A method as described in claim 16, further comprising preceding the telephone number with an access code of the selected telephone carrier.
 18. A method as described in claim 14, further comprising:determining whether a telephone number encoded by the dialing signals is allowed; and forwarding the dialing signals to the central office only if the telephone number is allowed.
 19. A method as described in claim 14, wherein the step of charging the capacitor comprises connecting the capacitor to a d.c. voltage that has the same bias polarity as the telephone link to which the capacitor will be connected.
 20. A method as described in claim 14, further comprising only switching the a.c. load across the telephone link to attenuate the dialing signal and disconnecting the a.c. load at other times.
 21. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link across tip and ring lines, the device comprising:a switchable a.c. load selectively connectable across the tip and ring lines for attenuating dialing signals transmitted on the tip and ring lines; a dialing signal detector for identifying dialing signals across the tip and ring lines of the telephone link; a tone generator for generating dialing signals on the telephone link; a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link, and for controlling the tone generator to transmit dialing signals on the telephone link; and a shared capacitor for coupling the tone generator and the dialing signal detector to the tip line.
 22. A device as described in claim 21, further comprising an equalizing network for enabling the dialing signal detector to detect dialing signals across the switchable load whether or not the switchable load is connected across the tip and ring lines.
 23. A device as described in claim 22, wherein the equalizing network presents dialing signals across tip and ring lines to the dialing signal detector at substantially the same magnitude whether or not the switchable a.c. load is connected across the tip and ring lines.
 24. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link across tip and ring lines, the device comprising:a switchable a.c. load selectively connectable across the tip and ring lines for attenuating dialing signals transmitted on the tip and ring lines; a dialing signal detector for identifying dialing signals across the tip and ring lines of the telephone link; a tone generator for generating dialing signals on the telephone link; a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link, and for controlling the tone generator to transmit dialing signals on the telephone link; an interfacing capacitor for coupling the tone generator and the dialing signal detector to the tip line; and an equalizing network for enabling the dialing signal detector to detect dialing signals across the switchable load whether or not the switchable load is connected across the tip and ring lines.
 25. A device as described in claim 24, wherein the equalizing network presents dialing signals across the tip and ring lines to the dialing signal detector at substantially the same magnitude whether or not the switchable a.c. load is connected across the tip and ring lines.
 26. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link across tip and ring lines, the device comprising:a switchable a.c. load selectively connectable across the tip and ring lines for attenuating dialing signals transmitted on the tip and ring lines; a dialing signal detector for identifying dialing signals across the tip and ring lines of the telephone link; a tone generator for generating dialing signals on the telephone link; and a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link, and for controlling the tone generator to transmit dialing signals on the telephone link, wherein the controller charges a capacitor of the a.c. load by connecting it across the tip and ring lines in response to an off-hook condition and then disconnects the capacitor to allow a dial tone to reach a user at the telephone.
 27. A device as described in claim 26, wherein the controller connects the a.c. load to the telephone link to prevent all but the first part of the first dialing signal from reaching the central office servicing the telephone link.
 28. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link across tip and ring lines, the device comprising:a switchable a.c. load selectively connectable across the tip and ring lines for attenuating dialing signals transmitted on the tip and ring lines; a dialing signal detector for identifying dialing signals across the tip and ring lines of the telephone link; a tone generator for generating dialing signals on the telephone link; and a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link, and for controlling the tone generator to transmit dialing signals on the telephone link, wherein the controller detects the dialing signals, accesses a list of speed-dialing numbers in response to the detected dialing signals, and controls the tone generator to transmit dialing signals corresponding to the accessed numbers to a central office controlling the telephone link.
 29. A system for transmitting information over telephone wiring, comprising:a detector for identifying dialing signals across the tip and ring conductors of the telephone wiring; a switchable load selectively connectable to the tip line for attenuating the dialing signals to prevent action by a central office connected to the telephone wiring; and a controller for controlling the switchable load and interpreting dialing signals attenuated by the load as commands for controlling non-telephonic electronic devices.
 30. A system as described in claim 29, wherein the system affects an appliance in response to the commands.
 31. A system as described in claim 29, wherein the system provides an intercom message in response to the commands.
 32. A system as described in claim 31, wherein the message is prerecorded.
 33. A system as described in claim 31, wherein the message is a voice or other a.c. signal transmitted over the telephone wiring.
 34. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link having tip and ring conductors, the device comprising:a dialing signal detector for identifying dialing signals across the tip and ring conductors of the telephone link; a switchable a.c. load connectable across the tip and ring conductors for attenuating dialing signals transmitted across the tip and ring conductors, the a.c. load having a capacitor that is charged prior to being connected across the tip and ring conductors; and a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link.
 35. A dialing signal detection and transmission-inhibiting device adapted to be connected in parallel with a telephone on a telephone link across tip and ring lines, the device comprising:a switchable a.c. load connectable across the tip and ring lines for attenuating dialing signals transmitted on the tip and ring lines, a capacitor of the a.c. load being charged prior to being switched across the tip and ring lines in response to detection of the dialing signal; a dialing signal detector for identifying dialing signals across the tip and ring lines of the telephone link; tone generator for generating dialing signals on the telephone link; a controller for connecting the a.c. load to attenuate the dialing signals in response to detection of a first dialing signal by the dialing signal detector to thereby prevent action by a central office servicing the telephone link, and for controlling the tone generator to transmit dialing signals on the telephone link. 